Hello to everybody

I am an new user of Haskel and generally in functional programming and I could
say that I am very impressed from this Language. Though I can't understand the
use of datatypes.

Let's take a firly simple situtation

e.g. data Pair a b = Pair a b

i.e. an new type with name Pair parameterized over the types a,b with one
Constructor named Paid which take two values of type a,b

a more complex one would be
data Either a b = Left a | Right b

i.e a new type named Either parameterized over the types a, b and two
Constructors 1. Left which take one value of type a and 2. Right which takes one
value of type b

I consider that the definitions above are well formulated. Nevertheless I can't
understand them quite well.

I have read that datatypes are used to define new structures. So, is there any
corresponding example in C, sinch I am quite familiar with structures in it? I
hope the word C here is allowed here :o)